The stroke length of the tamper bar of a tamper in a screed must be changed, e.g., depending on the laying thickness or other laying parameters. Normally, this is carried out such that, during a shutdown, the eccentric drive mechanism of the tamper bar is exposed and the eccentric bushing, which is fixed in position on the eccentric section, is released by means of tools, manually rotated relative to the eccentric section and fixed in position once more. This has the effect that the sum of the eccentricities of the eccentric section and of the eccentric bushing, which are effective in the direction of stroke of the tamper bar, change and, consequently, the stroke length changes as well. This procedure is cumbersome and time-consuming, since a screed has normally arranged therein a plurality of tampers, e.g., in an extending screed at least four tamper bars and eight connecting rods.
EP 2 325 392 discloses a tamper in the case of which the stroke length of the tamper bar is infinitely variable in a remotely controlled manner via a gear mechanism during laying without any change in the direction of rotation of the drive shaft, the gear mechanism engaging between the eccentric bushing and the eccentric section. A manual adjustment of the stroke length of the tamper bar is thus no longer necessary.
The tamper according to EP 2 325 391 B1, which represents a tamper of the type in question, allows a change in the stroke length of the tamper bar without any tools being necessary, said change being accomplished by a reversal of the direction of rotation of the drive shaft. The eccentric bushing and the eccentric section have provided between them a driver and a curved track with end stops for the driver, the stops being, when seen in the direction of rotation, spaced apart at a distance that is larger than the circumferential length of the driver. When the reversal of the direction of rotation takes place, the eccentric bushing is rotationally displaced relative to the rotating eccentric section e.g., due to the moment of inertia of the eccentric bushing and due to the reaction forces resulting from the compacting effect of the tamper bar, until the driver, after having been moved away from one of the end stops, moves into contact with the other end stop. The two end stops define different relative rotational positions between the eccentric bushing and the eccentric section, at which different stroke lengths of the tamper bar result from the different sums of the eccentricities of the eccentric section and of the eccentric bushing in the direction of stroke of the tamper bar, e.g., 4.0 mm at one of the relative rotational positions and 8.0 mm at the other relative rotational position. Although the tamper can be changed over without any tools, it is characterized by a simple structural design in comparison with the driving devices provided between the eccentric bushing and eccentric section in the tamper according to EP 2 325 392 A. In particular embodiments operating with low mass and/or friction and/or compacting forces, do not allow the changeover principle to be used with sufficient operational reliability.